Steel Product with Improved Weathering Characteristics in Saline Environment

ABSTRACT

The present invention is related to a steel product of the weathering steel type, i.e. a product exhibiting increased corrosion resistance after having been exposed to the outside environment for a given time. According to the invention, the steel product is defined by a composition consisting of (in wt %):—Carbon between 0.03 and 0.2,—Manganese between 0.5 and 2,—Copper between 0 and 0.5,—Ti between 0 and 0.1,—Cr between 0 and 0.5,—Nickel between 0 and 0.2,—Niobium between 0 and 0.1,—Nitrogen between 0 and 0.01,—Sulphur between 0 and 0.01, Phosphor between 0 and 0.01,—Aluminium higher than 0 and maximum 1.5,—Silicon between 0.25 and 1.5, the balance being Fe and accidental impurities, and wherein the sum of the Al and Si levels is higher than about 0.85 wt %.

FIELD OF THE INVENTION

The present invention is related to steel types and products made thereof, which exhibit an increase in corrosion resistance when exposed to a chlorine-rich environment. These steels are known as weathering steels.

State of the Art

Weathering steels have been studied and documented for some time. The corrosion resistance is caused by a layer of rust that forms on the steel surface when it is exposed to the outside environment. Traditional alloying elements which enhance this type of weathering resistance are Cu, P, Cr and Ni. However, traditional weathering steels haven proven to be ineffective in an environment containing Cl ions, i.e. marine or seaside environments. It was observed that the Cl ions destabilize some of the stable components that form the protective oxide layer. Solutions to this problem have been proposed, mainly in the form of changes in the steel composition. The best results have been achieved so far by applying significantly higher levels of Ni. However, given that a high amount of Ni is needed and taking into account that the price of Ni is a highly critical parameter in the commercial exploitation of steels, this solution is not regarded as ideal, and a need has arisen for an alternative to the Ni-solution.

One solution is described in JP2006118011, in the form of a steel composition comprising C, Al, Si, P, Ni, Cu, N, with Sn and/or Sb added in an amount between 0.03 and 0.5 wt % and one or more of Ti, Nb, Mo, W, V, Ca or Mg, with the mass ratio of Ni/Cu smaller than or equal to 0.5. The weathering capability in saline environment is mainly obtained by the presence of Sb and/or Sn, which stabilize the amount of Fe2+ ions when the steel is in a wet and saline environment, by reacting with Fe3+ ions, produced by oxidation of Fe2+. However, the use of Sn and/or Sb is disadvantageous for various reasons :

-   -   both Sn and Sb have a detrimental effect on hot ductility as         they decrease the solubility of Cu in austenite at high         temperature     -   Sb has a detrimental effect on weld toughness     -   Sn can not be removed from the steel during refining     -   Sb causes a risk of forming Stibine (SbH3)—a toxic and flammable         compound.

AIMS OF THE INVENTION

The present invention aims to provide a weathering steel with a Ni-content in the same order of magnitude as in traditional weathering steels, and with an increased corrosion resistance in marine or seaside conditions, as well as with excellent mechanical properties.

SUMMARY OF THE INVENTION

The invention is related to a steel product as disclosed in the appended claims. The invention is in particular related to a steel product as disclosed in the main independent claim, suitable for use in outdoor environments, in particular in saline conditions. The steel product of the invention may be any type of construction element such as a steel sheet or beam. In particular, the invention is related to a steel composition characterized by the following contents (in wt %):

-   -   Carbon between 0.03 and 0.2,     -   Manganese between 0.5 and 2,     -   Copper between 0 and 0.5,     -   Ti between 0 and 0.1,     -   Cr between 0 and 0.5,     -   Nickel between 0 and 0.2,     -   Niobium between 0 and 0.1,     -   Nitrogen between 0 and 0.01,     -   Sulphur between 0 and 0.01,     -   Phosphor between 0 and 0.01,     -   Aluminium higher than 0 and maximum 1.5,     -   Silicon between 0.25 and 1.5,         the balance being Fe and accidental impurities, and wherein the         sum of the Al and Si levels is higher than about 0.85 wt %.

According to a preferred embodiment, the following narrower ranges are defined for Cu, Ti, Cr, Ni and Nb (in wt %):

-   -   Copper between 0.2 and 0.4     -   Ti between 0.01 and 0.07,     -   Cr between 0.2 and 0.5,     -   Nickel between 0.1 and 0.2,     -   Niobium between 0.02 and 0.1.

According to the preferred embodiment, the steel product according to the invention comprises no Sn and no Sb.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is related to a steel product of the weathering steel type, i.e. a product exhibiting increased corrosion resistance after having been exposed to the outside environment for a given time.

According to the invention, the steel product is defined by a composition consisting of (in wt %):

-   -   Carbon between 0.03 and 0.2,     -   Manganese between 0.5 and 2,     -   Copper between 0 and 0.5,     -   Ti between 0 and 0.1,     -   Cr between 0 and 0.5,     -   Nickel between 0 and 0.2,     -   Niobium between 0 and 0.1,     -   Nitrogen between 0 and 0.01,     -   Sulphur between 0 and 0.01,     -   Phosphor between 0 and 0.01,     -   Aluminium higher than 0 and maximum 1.5,     -   Silicon between 0.25 and 1.5,         the balance being Fe and accidental impurities, and wherein the         sum of the Al and Si levels is higher than 0.85 wt %. The word         ‘between’ as used above is to be understood as inclusive of the         borders of the range in question.

According to a preferred embodiment, the amounts of Cu, Ti, Cr, Ni and Nb are as follows (in wt %):

-   -   Copper between 0.2 and 0.4     -   Ti between 0.01 and 0.07,     -   Cr between 0.2 and 0.5,     -   Nickel between 0.1 and 0.2,     -   Niobium between 0.02 and 0.1,         This embodiment combines good weathering behaviour in saline         conditions with excellent mechanical properties, due to the         minimum amounts of the abovenamed elements (Cu, Ti, Cr, Ni, Nb).

The present invention reveals that the inventive steel compositions favour the formation of fine grained goethite (α-FeOOH) in the corrosion layer forming on the surface of a steel product having these compositions, due to the specific combined Al and Si content. With ‘fine grained goethite’ is meant goethite with grain sizes from about 8 nm to about 15 nm. The formation of such fine grained goethite is beneficial for the stability of the oxide layer in saline conditions.

It is to be noted that the inventive compositions do not necessarily include an addition of significant amounts of the traditional elements regarded as beneficial for weathering resistance in saline conditions, such as Ni. The inventive compositions do not comprise Sn and Sb above an accidental impurity level. Preferably, the steel product of the invention comprises no Sb or Sn. Elements such as Nb, Ti, Cu, Cr may be added for improving the mechanical properties of the steel, but are not required for ensuring the weathering behaviour in saline conditions.

The invention is related to any type of steel product having the composition as claimed, e.g. steel sheets or beams, obtained by any standard steel-making process, e.g. hot-rolled or cold-rolled sheets or beams.

EXAMPLES

Table 1 shows the compositions of 3 test samples S1 to S3 having Al and Si contents according to the invention, as well as 2 comparative samples C1 and C2 having Al and Si contents falling outside the claimed ranges and/or wherein the combined Al and Si content is less than 0.85 wt %.

The test samples have been obtained by standard steel processing methods, involving at least a hot rolling step, possibly followed by a cold rolling step. The samples were subjected to a dip-dry test during 60 days, using a 0.05M NaCl solution, in order to test weatherability behaviour in saline conditions. As a result, oxide layers were formed on the samples. The composition of these oxide layers was investigated by Mössbauer spectroscopy, used in particular to measure the relative area of superparamagnetic goethite in the oxide layer. Superparamagnetic goethite has a particle size between 8 and 15 nm, as opposed to antiferromagnetic goethite, having particle sizes above 15 nm.

Table 2 clearly shows that the combined Al and Si contents according to the present invention give rise to a considerable increase in the relative area of fine-grained goethite in the oxide layer.

TABLE 1 example compositions (in wt %) Sample C Si Al P Mn S N Ti Mo Cu Ni Cr S1 0.19 0.35 1.2 0.077 1.6 0.006 0.005 0.007 0.025 0.02 0.025 0.025 S2 0.07 1 0.03 0.02 0.5 0.003 0.003 — — 0.3 — — S3 0.07 0.25 0.8 0.02 0.5 0.003 0.003 — — 0.3 — — C1 0.07 0.25 0.4 0.02 0.5 0.003 0.003 — — 0.3 — — C2 0.03 0 1 0.01 1.8 0.003 0.007 — — 0.4 — —

TABLE 2 Relative area of superparamagnetic goethite (in %) Sample % S1 51 S2 40 S3 29 C1 15 C3 16 

1-4. (canceled)
 5. A steel product consisting of (in wt %): Carbon between 0.03 and 0.2 Manganese between 0.5 and 2 Copper between 0.2 and 0.4, Titanium between 0.01 and 0.07, Chromium between 0.2 and 0.5, Nickel between 0.1 and 0.2, Niobium between 0.02 and 0.1, Nitrogen between 0 and 0.01, Sulphur between 0 and 0.01, Phosphor between 0 and 0.01, Aluminium higher than 0 and maximum 1.5, Silicon between 0.25 and 1.5, the balance being Fe and accidental impurities, and wherein the sum of the Al and Si levels is higher than about 0.85 wt %.
 6. The steel product according to claim 5, comprising no Sn and no Sb.
 7. A steel product according to claim 5, said product being a sheet or beam.
 8. A steel product according to claim 6, said product being a sheet or beam.
 9. Use of a steel product according to claim 5 as a product that exhibits an increase in corrosion resistance when exposed to a chlorine-rich environment.
 10. Use of a product according to claim 6, in saline outdoor conditions. 